assert: ensure .rejects() disallows sync throws

This updates `assert.rejects()` to disallow any errors that are thrown
synchronously from the given function. Previously, throwing an error
would cause the same behavior as returning a rejected Promise.

Fixes: #19646
PR-URL: #19650
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/assert.js

@@ -451,8 +451,11 @@ async function waitForActual(block) {
  if (typeof block !== 'function') {
    throw new ERR_INVALID_ARG_TYPE('block', 'Function', block);
  }

  // Return a rejected promise if `block` throws synchronously.
  const resultPromise = block();
  try {
    await block();
    await resultPromise;
  } catch (e) {
    return e;
  }

test/parallel/test-assert-async.js

@@ -13,7 +13,7 @@ common.crashOnUnhandledRejection();

(async () => {
  await assert.rejects(
    () => assert.fail(),
    async () => assert.fail(),
    common.expectsError({
      code: 'ERR_ASSERTION',
      type: assert.AssertionError,
@@ -57,4 +57,17 @@ common.crashOnUnhandledRejection();
      }
    );
  }

  {
    const THROWN_ERROR = new Error();

    await assert.rejects(() => {
      throw THROWN_ERROR;
    }).then(common.mustNotCall())
      .catch(
        common.mustCall((err) => {
          assert.strictEqual(err, THROWN_ERROR);
        })
      );
  }
})().then(common.mustCall());